This invention relates to articles or article components having improved combinations of in-plane shear modulus (IPSM) and circumferential tensile modulus which increases resistance to various stresses that arise during use of the article. More particularly, this invention relates to rubber articles, reinforced with non-metallic multifilaments, which are subjected to tensile and shear stresses in use, such as those found for example in belts for tires, and particularly radial tires, and transmission drive belts.
Tires are high performance composites that must: (1) develop longitudinal (circumferential) forces for acceleration and braking; (2) develop lateral forces for cornering; (3) support a vertical load; and (4) provide cushioning. Thus, the tire belt serves to provide stiffness to the tire and thereby contributes significantly to cornering characteristics, footprint deformation (contact with road) and forward motion. Increasing the tensile modulus around the circumference of the tire enhances the efficiency in transmitting the driving force from the axle to the tire and ultimately to the road. As the driver steers the vehicle, cornering forces are generated which subject the contact patch (tire area in contact with a surface, or footprint region), and thus the belt, to in-plane shearing forces. High rigidity of the tire belt allows the tire tread in the footprint region to remain flat and in contact with the road, thereby enhancing cornering and treadwear. The importance of the belt makes it a target for improvement for use in high performance tire applications.
The primary concern for agricultural and off-road tires is how efficiently power is transmitted through the tire. This performance goal is largely impacted by the tire belt's circumferential (or longitudinal) modulus. A high circumferential tensile modulus in the tire belt translates into a high degree of transmitting power from the rotation of the wheel to the forward movement of the vehicle. In maximizing this property of a belt, however, the property of in-plane shear modulus is adversely affected. For passenger tires, that is, tires for use in passenger vehicles in normal day-to-day use, in addition to power transmission, one is very interested in ride comfort, handling and treadwear. Ride comfort is influenced by the out-of-plane bending of the tire belt. Lower out-of-plane bending modulus allows the tire tread to readily envelope road obstructions thereby minimizing transmission of vertical deflection to the tire axle. Handling, such as cornering, is impacted by the tire belt's in-plane bending. As the in-plane shear modulus of a tire belt increases, handling response improves as well. However, one must maintain a desirable level of power transmission therefore the circumferential modulus cannot be allowed to decrease below acceptable levels when optimizing this characteristic. Belt design for passenger tires is therefore different than that for off-road and agricultural tires in its need to optimize a greater number of belt parameters to achieve desirable performance goals.
Angle-ply belt composites for pneumatic tires are typically made by stacking in alternate directions two or more plies of filament-reinforced rubber sheets. The reinforcing filaments are typically unidirectional within a sheet. During the incorporation of these unidirectional plies into the tire, an angle is formed between the reinforcement filaments and the tire circumferential line. This angle is typically 20° to 23°. This conventional manufacture of belt composites yields a belt with cut filament edges that are located along the entire circumferential length of the belt edge. Thus, the individual reinforcing filaments of the angle-ply composite are disconnected, which detracts from the mechanical and fatigue properties of the composite due to the ability of the cut filament ends to undergo independent rather than collective movement. Also, cut cord ends represent material discontinuities resulting in undesirable stress concentrations.
Use of steel wire cord for the reinforcing filament is the most common practice in conventional tire belts. This is so because steel cord has compressive and tensile properties adequate for belt reinforcement. However, due to its low tenacity and high density the weight of steel is a drawback which adversely affects fuel economy. In addition, for optimum performance at high speeds, steel-reinforced belts typically require the use of cap plies, or overlays, wherein low density synthetic filaments overlap the cut steel cord edges thus helping to contain the weighty steel cords and to reduce stress-concentration at the sharp cut edges of the steel cord, thereby extending both tire life and high speed capabilities. The use of cap plies would not be necessary in a tire made with a synthetic filament-reinforced belt, representing a savings in both labor and material costs. In addition, the use of steel-reinforced belts makes tire retreading impractical if the steel is corroded. Further, tire recycling of steel-belted tires is more difficult (due to excessive wear of tire shredding equipment) and generates a high percentage of low-grade crumb rubber (i.e. not guaranteed metal free). Overall steel-belted tire recycling is less cost effective than recycling of synthetic organic filament-belted tires.
Given the many drawbacks of the use of steel for reinforcing tire belts, it is highly desirable to replace steel with lightweight materials as reinforcement. Advantageously, the tensile strength of lightweight, synthetic filaments such as PEN, PET, aramid and nylon, is much better than steel's tensile strength when compared at a given fiber weight. As tires are generally designed to strength, this difference results in less cord per tire when synthetics are used. Disadvantageously, however, such synthetic fibers generally have lower compressive moduli than steel wire, and thus yield composites with lower in-plane shear moduli. The lower in-plane shear modulus of a tire belt is detrimental to both the cornering coefficient and treadwear characteristics of a pneumatic tire.
There are several possible approaches for augmenting belt performance when reinforced with synthetic fibers. Generally these approaches introduce additional plies to the belt and/or a third dimension of reinforcement. The latter approach may include: tacking together the plies of the belt, by stitching the plies together, folding plies, or braiding or interweaving.
U.S. Pat. No. 3,616,832, U.S. Pat. No. 3,854,515 and International Publication WO 98/14336 seek to replace steel cord in tire belts with synthetic material. U.S. Pat. No. 3,616,832 teaches four ply belts with ply angles of 50 to 35°; examples employ 15° ply angles. U.S. Pat. No. 3,854,515 teaches the use of four plies in a belt, all with 30° ply angles to replace a steel belt. The polyester used for reinforcement has a lower polymerization degree than is used in conventional polyester cord in combination with twist restrictions. International Publication WO 98/14336 teaches the use of polyester with particular cord constructions in belts designed for use in radial carcass tires for heavy duty use, such as off-road tires and agricultural uses. The examples utilize polyethylene terephthalate (PET) fiber and teaches conventional ply angles of 20° in a four ply belt and 15° to 30°, preferably 17° to 23°, in a two ply belt. All three disclosures teach the use of conventional ply angles with synthetic reinforcement material and do not contemplate the lower in-plane shear modulus problem inherent in these tire belts compared to those reinforced with steel.
The use of altered bias ply angles has been applied in the design of drive belts. U.S. Pat. No. 5,211,609 teaches a three ply composite drive belt with two layers having bias plies of approximately 45° to 75° (preferably 70°) with respect to the longitudinal axis of the belt. The bias angles are chosen to balance the lateral forces that affect belt tracking. There is no teaching on the composition of the reinforcing cable, nor is there the suggestion of applying the construction to tire belts.
The use of stitching in laminate composite design has been disclosed, however none of the disclosures teach the beneficial effect on the in-plane shear modulus, or teach or suggest the use of stitching in a tire belt composite. The use of stitching in a composite laminate structure is taught in U.S. Pat. No. 4,331,495 and commonly-assigned U.S. Pat. Nos. 5,185,195; 5,198,280; and 5,591,933. Disclosure '495 has no teaching on the disposition of the reinforcing filaments in adjacent plies, teaches a different stitching pattern, is not intended for a flexible elastomeric composite and does not teach or suggest the benefit of stitching with respect to in-plane shear modulus. Disclosures '195 and '280 employ stitching to secure layers of a penetration resistance article with at least two adjacent paths of stitching being less than 0.125 inch apart. Disclosure '933 teaches a slack stitching process to achieve a desirable level of delamination in a penetration resistant article. “Mechanical Properties of 3-D Composites” by M. Cholakara, B. Z. Jang and C. Z. Wang (ANTEC'89, pp. 1549-1551) teaches the effects of stitching laminates of Kevlar and epoxy resin on damage tolerance by improving interlaminar shear strength. It does not teach unidirectional fiber, teaches no specifics of stitching, does not suggest the use of stitching in tire belt composites, and does not consider the in-plane shear modulus.
Composites employing a folded ply have been taught, for example, in U.S. Pat. No. 5,535,801 which teaches the use of a aromatic polyamide fiber-reinforced ribbon which is 5 to 15 mm wide and is folded to zigzag between the edges of the belt. The patent is silent on cord construction and cord properties for the reinforcement cord, and no specific ply angles are taught. Although the zigzag belt would contain continuous fiber reinforcement with cut cords only at the two ends of the ribbon, it would possess many seams and would be laborious to prepare given the narrow width of the ribbon. U.S. Pat. No. 4,210,189 teaches a belt formed by folding a single, wide ply in which the reinforcement cords traverse the width of the single ply into a structure having three folds and four superimposed plies. Cut cord ends are at either edge of the width of the single unfolded ply and when folded, the cut cord ends are present throughout the circumference of the folded belt structure. The patent is silent on cord construction and cord properties for the reinforcement cord. For use in a radial tire, belt ply angles taught are 0° to 30° and for use in a bias tire, belt ply angles taught are 20° to 55°. U.S. Pat. No. 3,830,276 also teaches a folded belt in which the reinforcement cords are cut and the cut ends are present throughout the circumference of the folded belt structure. A braided structure is taught, for example, in U.S. Pat. No. 4,830,781 which discloses a woven tire reinforcing component for use underlying the thread and at least the sidewall regions of a pneumatic tire. The woven structure is made using a coated continuous cord reinforcement preferably containing a single cord spaced within a rubber coating. The patent is silent on cord construction and cord properties for the reinforcement cord.
Other teachings of composites utilizing polyethylene naphthalate fiber include: Japanese Publication Number 30210-1997 (Feb. 4, 1997); Japanese Publication Number 276704-1996 (Oct. 22, 1996); Japanese Publication Number 310251-1995 (Nov. 28, 1995); Japanese Publication Number 193608-1997 (Jul. 29, 1997); and Japanese Publication Number 142101-1997 (Jun. 3, 1997), and International Publication WO 98/47726.
After extensive research into composite design, this invention resulted in improving the conventional tire belt design by employing only non-metallic reinforcement such that a significantly lighter weight belt was able to have comparable or superior circumferential tensile modulus and in-plane shear modulus when compared to a steel belt. This combination of properties resulted from combining the properties of the synthetic filament cords with novel composite architectures. The critical cord properties include initial tensile modulus and initial compressive modulus.